There are numerous prior art disclosures illustrating the assembly of multiple circuit boards. Among these are teachings of methods to assemble multiple circuit boards employing solder paste as well as methods of assembling circuit boards using various attachment mechanisms including flexible interconnect embodiments and various assemblies employing conventional hardware such as screws or snapping devices.
FIG. 1 is a diagram of a prior art Solder Interconnect Assembly design. A typical disclosure of such a device is given in U.S. Pat. No. 5,471,368 issued to Downie et al. (Downie). FIGS. 2 and 3 illustrate the individual mother and daughter boards that are soldered together in such an embodiment.
FIG. 2 is an illustration of a prior art Mother Board Design layout. The mother board layout is designed to incorporate the same test features as with the mechanical boards. This includes continues monitoring of continuity during each test and determination of actual interconnect resistance. This board will be a single sided 0.047 inch thick FR-4 board with 1 ouncecopper traces. Solder mask will cover all traces except the interconnect geometries and test pads. Surface finish over exposed pads will be Hot Air Solder Leveled (HASL) as this will most probably be used in a production mother board. The diagram in FIG. 2 represents the mother board layout.
FIG. 3 is a Scalloped Edge Daughter Board Design layout. The daughter boards are designed to provide electrical continuity throughout the test vehicles when soldered to the mother boards. The boards will be double sided 0.047 inch thick FR-4 with 1 ounce copper traces. All conductors will be plated with soft wirebonable gold over nickel. Plate through holes located at the board edge will be routed through to form edge receptacles (scallops) for the solder. Solder mask will cover all traces except the interconnect geometry. FIG. 3a and FIG. 3b represents the daughter board layout. FIG. 3a shows the secondary side and FIG. 3b shows the primary side.
A problem exists within the prior art that is especially apparent in assemblies used to house semiconductor based image sensors. These image sensors are sensitive to high temperatures. Therefore, the solder based assembly described above is difficult to assemble without damage to the image sensor device. Certain electronic components such as charge-coupled devices (CCDs) or other imaging sensors, have low temperature requirements. These devices can not be assembled to printed circuit boards using traditional re-flow techniques primarily due to the temperature sensitivity of microlenslets and Color Filter Arrays currently used. As sensor packaging migrates to more integrated imaging heads, there is a need for a relatively low temperature, high volume, inexpensive process to attach an imaging head substrate to a product driver board.
Typically tests vehicles for solder assemblies will use hot nitrogen to reflow the solder during the interconnect process. Screen paste will be stenciled down onto the mother board and reflowed using standard SMT methods. Four stencil layouts with different sized openings will allow tests to determine the proper solder volume for quality solder joints. The mother board will be placed onto a fixture plate followed by the daughter board. Hot nitrogen will be directed to the solder pads only through a custom nozzle. This will cause the solder to reflow and wick up the scalloped edges on the daughter board completing the electrical interconnect. Flux may have to be applied prior to reflow to ensure quality solder joints. This is a time consuming method for assembly of mother/daughter board configurations.
Other assemblies employ flexible interconnects, such as U.S. Pat. No. 5,007,842, issued to Deak et al (Deak). This prior art device is appropriate to use for solid state image sensor assemblies in terms of high temperature protection, however requires plates with slots and coil springs while makes the assembly costly.
From the foregoing discussion is should be apparent that there remains a need within the art for cost effective assembly that can effectively attach mother and daughter boards that provides a low temperature solution with high volume capabilities.